The limited supply of fossil fuels and the problems that arise upon combustion of such fuels have caused intensive research in order to find alternative energy sources, such as wind, water, solar and nuclear energy. Using solar cells to convert solar radiation energy into electric energy is a promising method for achieving emission-free and renewable production of electricity.
Solar cell technologies can be divided into first, second or third generation. First generation solar cells are based on silicon. These solar cells have quite high degree of efficiency. However, the supply of the required high-quality silicon is at present limited and not sufficient for the fast growth of the industry. Moreover, the silicon solar cells are too expensive to manufacture and are therefore not yet close of being economically competitive with conventional sources of energy.
The solar cells of the second generation are so-called thin layer solar cells, e.g. CIS or CIGS solar cells. Using this type of solar cells allows reducing the material consumption and thus the manufacturing costs, since the layer of the semi-conducting material is very thin (<1 μm). The layer is applied directly on a substrate by e.g. vaporization. The size of the produced module is therefore no longer limited by the size of the silicon wafer sheet, and the module can thus be widely varied depending on the device design and the size of the glass sheet of choice. Additionally, very high efficiencies have been obtained. The drawbacks of the technology are the extremely high investment cost for initiating production, and the difficulty to reduce the costs to a level where the products financially may compete with conventional energy sources.
The third generation solar cells may be defined as the ones being in the R&D phase, with the ambition of lowering the costs for producing electricity from solar energy. One of the most-promising technologies of the third generation solar cells are photo-electrochemical systems, also known as dye-sensitized solar cells. These cells comprise a nanoporous semi-conducting material that is soaked with a light-absorbing dye, a counter electrode and an electrolyte. The potential of photo-electrochemical systems is defined by especially three factors: a low investment cost for initiating production, a lack of significant cost barriers that cannot be broken, and a flexible design, allowing manufacturing of devices of different size. Despite that the efficiencies of dye-sensitised solar cells still are lower than the ones obtained with the first and second generation solar cells, there is a high potential that the overall costs for generating electricity from solar cells may become competitive with conventional energy sources.
Monolithic electrochemical systems are photo-electrochemical systems where the working electrode and the counter electrode are assembled in a single integrated body, and are previously known in the art. The working electrode and the counter electrode are separated by means of an intermediate layer of a porous insulating material. The working and the counter electrodes are made of porous structures and an electrolyte is at least partially filled in the porous structure, which is a monolith comprising a layer forming the working electrode, a layer forming the counter electrode and an insulating layer separating the counter electrode from the working electrode.
An early example of a monolithic photo-electrochemical system is disclosed in WO97/16838 that describes a battery of photovoltaic cells consisting of a monolithic assembly of a plurality of serial-connected photovoltaic cells. Traditionally photo-electrochemical systems include a first substrate provided with a first electrode and a second substrate provided with a second electrode. The first and second substrates are positioned so that the electrodes are facing each other and separated by a small gap in between. In order to ensure that the gap is of a magnitude within a desired range, a spacer can be used to separate the substrates by a certain distance. The system is sealed at the edges of the first and second substrate and between adjacent cells to prevent the electrolyte from making connections between the cells, and/or to avoid unwanted contact between the electrolyte and current collectors of the cells. In order to create photo-electrochemical systems having uniform properties over the entire active area of the system, it is essential that the distance between the electrodes is kept within a narrow range, which aggravates production. Another important aspect to keep in mind is that the electrolyte should be prevented from making contacts between adjacent serial-connected cells. The horizontal and vertical positioning of the two substrates must thus be made with very high precision, which also makes production difficult. A further drawback with this traditional, bilithic type of photo-electrochemical systems is that the electrolyte normally is introduced after assembly of the system. The openings of the passages where the electrolyte is introduced must be well sealed after the introduction of the electrolyte to prevent the electrolyte from leaking and to protect the cell against penetration by water or impurities. Also, separate openings are required for each cell resulting in a large number of openings for a system with many cells. Besides, introduction of electrolyte through narrow passages into the essentially closed space between the substrates may lead to creation of air pockets in the system or to uneven distribution of electrolyte. All these factors make production difficult and may deteriorate the quality of the system.
Monolithic electrochemical systems allow very compact and simple design, eliminating the need for the first substrate being positioned at a specific distance from the second substrate. In this case, the electrochemical system can be constructed by applying a multi-layer structure to a substrate, after which the electrochemical system is closed. Electrolyte is preferably introduced before closing the electrochemical system. The structure can suitably be closed by means of a flexible sheet of at least one polymer layer, which is preferably applied to said structure in the presence of heat and sub-atmospheric pressure.
Experiments have shown that it is difficult to manufacture arrays of monolithic photo-electrochemical cells so that the cells have identical properties or properties within a desired range, even though they are manufactured simultaneously on the same substrate. A single photo-electrochemical cell is characterised by its current-voltage-characteristics. The current-voltage-characteristics for a cell vary with the light intensity and the light spectrum. Important parameters that describe the current-voltage-characteristics of a photo-electrochemical cell are the short-circuit current (Isc), the open-circuit voltage (Uoc), and the maximum power point (Pmax). The term fill-factor (ff) is often used to describe the curve as ff=Pmax/(Uoc*Isc). In order to reduce differences in the current-voltage-characteristics of individual cells arranged on a common substrate, the production requires high purity of the chemical components and clean production environment and production processes. Such measures lead to a much more expensive production. An important example is the necessity of having a perfect control of the deposition of the working electrode, the insulating layer and the counter electrode, in order to prevent the counter electrode from partially penetrating the insulating layer and by that touching the working electrode and/or the intermediate conducting layer on the substrate, causing energy losses and thus differences of the current-voltage characteristics of the individual cells on a common substrate. This becomes even more critical since a thin insulating spacer layer is desirable in order to obtain the best cell performance by facilitating the diffusion of the redox-couple electrolyte between the working and the counter electrodes. Another important example is the necessity of having a perfect control of the encapsulation procedure to avoid defects such as pinholes. Pinholes may create unwanted contact between the electrolyte and current collectors in the device leading to decreased efficiency and/or stability. It may also lead to electrolyte wandering from one cell to another, so-called electrophoresis. The problem with defects in the encapsulation becomes more difficult as the size of the solar device increases.
Since the technique for manufacturing monolithic photo-electrochemical systems is relatively inexpensive, the costs for production of large structures of solar cells are quite low. Considering the experimental experiences above implies, however, that the failure frequency of single cells is quite high. In the case where cells are serial-connected, the errant cells might work in negative direction, i.e. a negative voltage value is obtained for a given current value. If the decrease of performance is large due to the errant cells, the system must be discarded, which leads to decreased production yield and increased production costs. A reduced performance of the photo-electrochemical system may also be obtained due to different ageing of the cells and/or partial shading of the system, where in both cases, cells may work in negative direction since they produce too little current in relation to the other cells in the system.
A solution to this problem might be a disconnection of errant cells in a photo-electrochemical module system.
Disconnection of solar battery strings in case of ground fault is previously disclosed in the U.S. Pat. No. 6,593,520. The document describes a solar battery string formed by a plurality of series-connected solar panels, a detector for outputting an abnormality detection signal upon detection of a ground fault in the solar battery string, and at least one intermediate switch provided midway along the string that is shifted to an open state by the abnormality detection signal. Outputs from the solar battery strings are collected by the collector box comprising intermediate switches, string switches, ground fault abnormality detectors and the like. In other words, all the strings are connected to an external controlling device.